Mossbauer, Electron Paramagnetic Resonance, and Theoretical Studies of a Carbene-Based All-Ferrous Fe4S4 Cluster: Electronic Origin and Structural Identification of the Unique Spectroscopic Site
Mossbauer, Electron Paramagnetic Resonance, and Theoretical Studies of a Carbene-Based All-Ferrous Fe4S4 Cluster: Electronic Origin and Structural Identification of the Unique Spectroscopic Site
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DOI：
10.1021/ic802192w
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发表时间：
2009-04-06
影响因子：
4.6
通讯作者：
Bominaar, Emile L.
中科院分区：
文献类型：
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作者：
Chakrabarti, Mrinmoy;Deng, Liang;Bominaar, Emile L.
棕色固氮菌（Azotobacter vinelandii，Av 2）固氮酶铁蛋白的半胱氨酸配位[Fe 4S 4]簇可以达到全铁核氧化态。穆斯堡尔和电子顺磁共振（EPR）研究表明，全铁团簇具有基态自旋S = 4，自旋结构和Fe-57四极相互作用的有效3：1位置对称性。最近，Deng和霍尔姆报道了[Fe 4S 4（（Pr 2NHCMe 2）-N-i）（4）]（2），（1;（Pr 2NHCMe 2）-N-i = 1，3-二异丙基-4，5-二甲基咪唑-2-亚基）的合成，并表明全铁卡宾配位簇适合于物理化学研究。穆斯堡尔谱和EPR研究1，这里报道，揭示了该复合物的电子结构是惊人的相似的蛋白质结合的集群，这表明基态自旋和3：1的网站比的集群核心的自发扭曲的后果。为了深入了解1和Av 2中的全铁团簇的特殊基态的起源，我们研究了一个基于Heisenberg-Dirac-货车Vleck哈密顿量的理论模型，其交换耦合常数是Fe-Fe距离的函数。在简谐近似下，将交换能与弹性形变能相结合，得到了自旋S = 4的T-2畸变最小值和C-3v核结构，其中一个铁是唯一的，三个铁是等价的.这个最小值具有1和Av 2的全铁团簇的所有光谱和结构特征。我们的分析映射的独特的光谱铁网站的一个特定的网站在X射线结构的[Fe 4S 4]（0）核心在复杂的1和Av 2。
It is well established that the cysteinate-coordinated [Fe4S4] cluster of the iron protein of nitrogenase from Azotobacter vinelandii (Av2) can attain the all-ferrous core oxidation state. Mossbauer and electron paramagnetic resonance (EPR) studies have shown that the all-ferrous cluster has a ground-state spin S = 4 and an effective 3:1 site symmetry in the spin structure and Fe-57 quadrupole interactions. Recently, Deng and Holm reported the synthesis of [Fe4S4((Pr2NHCMe2)-N-i)(4)](2), (1; (Pr2NHCMe2)-N-i = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) and showed that the all-ferrous carbene-coordinated cluster is amenable to physicochemical studies. Mossbauer and EPR studies of 1, reported here, reveal that the electronic structure of this complex is strikingly similar to that of the protein-bound cluster, suggesting that the ground-state spin and the 3:1 site ratio are consequences of spontaneous distortions of the cluster core. To gain insight into the origin of the peculiar ground state of the all-ferrous clusters in 1 and Av2, we have studied a theoretical model that is based on a Heisenberg-Dirac-van Vleck Hamiltonian whose exchange-coupling constants are a function of the Fe-Fe distances. By combining the exchange energies with the elastic deformation energies in the harmonic approximation, we obtain for a T-2 distortion a minimum with spin S = 4 and a C-3v core structure in which one iron is unique and three irons are equivalent. This minimum has all of the spectroscopic and structural characteristics of the all-ferrous clusters of 1 and Av2. Our analysis maps the unique spectroscopic iron site to a specific site in the X-ray structure of the [Fe4S4](0) core both in complex 1 and in Av2.